Fix a typo

This PR fixes the following typo.

```diff
-mutliple
+multiple
```

spec/rubocop/cop/layout/multiline_method_argument_line_breaks_spec.rb

@@ -25,7 +25,7 @@
     end
   end
 
-  context 'when many arguments are on mutliple lines, two on same line' do
+  context 'when many arguments are on multiple lines, two on same line' do
     it 'adds an offense' do
       expect_offense(
         <<-RUBY
@@ -56,7 +56,7 @@
     end
   end
 
-  context 'when many arguments are on mutliple lines, three on same line' do
+  context 'when many arguments are on multiple lines, three on same line' do
     it 'adds an offense' do
       expect_offense(
         <<-RUBY
@@ -89,7 +89,7 @@
     end
   end
 
-  context 'when many arguments are on mutliple lines, three on same line' do
+  context 'when many arguments are on multiple lines, three on same line' do
     it 'adds an offense' do
       expect_offense(
         <<-RUBY